1. Field of the Invention
The present invention relates to the providing of an indication of the position of a movable member. More particularly, this invention is directed to apparatus for indicating the position of a movable member, for example a member which moves within a hermetically sealed enclosure without establishing pyhsical contact with such member and without requiring penetration of the wall of the enclosure. Accordingly, the general objects of the present invention are to provide new and improved methods and apparatus of such character.
2. Description of the Prior Art
While not limited thereto in its utility, the present invention is particularly well suited to the monitoring of the position of a control rod in a nuclear reactor. As is well known, in the interest of preventing leakage of primary loop coolant, portions of the structure of a nuclear reactor are sealed to the environment. Additionally, to minimize the possibility of exposing operating personnel to dangerous amounts of radiation, reactor control and particularly control rod positioning is commanded from a remotely located, central control station. In conventional practice with pressurized water type reactors, the reactor control rods are positioned within sealed housings which internally communicate with and extend from the reactor pressure vessel. Obviously, an important design requirement for a nuclear reactor is that the points of communication between the exterior and the interior of the pressure vessel, either directly or through the control rod housings, be minimized thereby minimizing potential leakage of pressure vessel coolant which, of course, is contaminated by radiation.
To further discuss the problem of control rod position monitoring as an example utilization of the present invention, a hermetically sealed control rod drive, which may either be hydraulic or include a drive motor, is to be preferred. However, with a hermetically sealed drive there is nothing extending out of the control rod housing or pressure vessel to provide an indication of where the rod is instantaneously positioned. With hermetically sealed drives, as well as in installations where a drive motor is positioned externally of the control rod housing and communicates with the rod drive through specially designed shaft seals, control rod position can not be accurately monitored without providing for the passage of a plurality of electrical conductors through the housing; such conductors providing communication between position sensing devices mounted on the control rod or its drive mechanism and the ambient atmosphere externally of the pressurized system. Additionally, the monitoring of control rod position in a nuclear reactor is further complicated by the fact that the control rod is typically submerged in the reactor coolant and, during operation of the reactor, the temperature of this fluid and thus of the rod itself becomes quite high. The design of position monitoring apparatus is thus complicated by the facts that, if portions thereof are to be physically affixed or connected to the control rod as has been past practice, such portions of the position indicating apparatus must be capable of withstanding high temperatures and must be able to operate while submerged in a fluid such as water.
Because of the above briefly discussed design requirements dictated by the operating environment and safety considerations, control rod position monitoring apparatus has previously employed mechanical or electromechanical components which were suitable for use within a control rod housing. The requirement for use of such mechanical or electromechanical components has, in turn, limited the flexibility of the design and in most cases required undesirable communication between the interior and the exterior of the control rod housing. As will be obvious to those skilled in the art, any failure of rod position sensing devices which would require a maintenance procedure would, with the failed component or components located within the pressurized system, necessitate a lengthy shut-down of the reactor.
In an attempt to overcome the problems incident to the use of position monitoring apparatus located either wholly or in part within the control rod housing or reactor pressure vessel, it has been proposed to mount radiation sources or magnetic field generators on the control rods or their drives and to position suitable radiation or field detecting devices on the exterior of the control rod housing. An example of a control rod position monitoring apparatus wherein magnetic field producing elements are mounted on the control rod drive may be found in U.S. Pat. No. 3,594,740 to J. Comeau and W. Zinn. While apparatus such as that disclosed in U.S. Pat. No. 3,594,740 overcomes many of the problems of prior art position sensors of the type which require some means for providing communication through the wall of the control rod housing, the patented device nevertheless possesses certain deficiencies. The principal one of these deficiencies is the large number of components, particularly the field sensing devices located externally of the control rod housing, required to obtain the desired accuracy. As is well known, the cost of fabrication and installation of a device is usually directly proportional to the number of components while reliability is usually inversely proportional to the number of components. Thus, to briefly summarize, those prior art devices for providing an indication of the position of a movable member which were suitable for use with nuclear reactor control rods have possessed the undesirable characteristics of either requiring sensing elements positioned within the control rod housing with the concurrent requirement for establishment of communication to such sensor elements through the wall of the housing and/or have required a substantial number of components with the corresponding inherent decrease in reliability and increase in cost.